A plasma CVD apparatus for forming a thin film on a substrate, an etching apparatus for etching a wafer by plasma when manufacturing a semiconductor integrated circuit, and the like have been widely used as a plasma processing apparatus. The plasma processing apparatus includes a vacuum chamber and an electrode plate disposed in the vacuum chamber. The electrode plate is formed of silicon or silicon carbide (SiC) and has a plurality of ventilation holes passing through the electrode plate in the thickness direction.
When processing a wafer by using the plasma processing apparatus, at first, the wafer is disposed in the vacuum chamber such that the electrode plate and the wafer face each other. Then, gas is supplied between the electrode plate and the wafer via the ventilation holes of the electrode plates. While maintaining this state, a high-frequency voltage is applied to the electrode plate; and thereby, plasma is generated between the wafer and the electrode plate.
In an etching apparatus, plasma is generated between the wafer and the electrode plate while supplying etching gas as the above-mentioned gas; and thereby, it is possible to etch the wafer. Recently, fine patterns are formed on a wafer with a high precision by using the plasma etching apparatus.
Such an electrode plate of a plasma processing apparatus is manufactured by forming through-holes (ventilation holes) with a diameter of 0.1 mm to 0.5 mm in a silicon substrate with a thickness of 4 mm to 5 mm, and the through-holes (ventilation holes) are formed by drilling, for example, as described in Patent Document 1.
In the case where the ventilation holes are formed with a high precision, the plasma processing apparatus including the electrode plate can uniformly supply gas. Accordingly, smoothness and working precision of the inner surfaces of the through-holes (ventilation holes) formed in the electrode plate affect the processing precision (wafer working precision) of the plasma processing apparatus. For example, in the case of a plasma etching apparatus, the working precision of a pattern formed on a wafer or the like is affected.
Patent Document 1 discloses that inner surfaces of through-holes (ventilation holes) are subjected to surface machining by a lapping machine or a polishing machine; and thereby, the ventilation holes with small surface roughness of the inner surfaces are formed.
Patent Document 2 discloses a method for forming through holes which includes a step of forming pilot holes at a relatively high speed by electrical spark machining or laser machining, and a step of machining inner surfaces of the pilot holes at a relatively low speed by using a diamond drill or the like to remove machining-damaged layers formed in the forming of the pilot holes. According to this method for forming through holes, it is possible to efficiently form fine holes with an excellent precision of worked surfaces.
Patent Document 3 discloses a method for forming ventilation holes with a high finishing precision which includes a step of forming pilot holes, and a step of irradiating a picosecond laser beam having a high energy density on inner surfaces of the pilot holes to remove thermally-affected zones formed in the forming of the pilot holes.
As described above, with regard to an electrode plate for a plasma processing apparatus such as a plasma CVD apparatus, an etching apparatus, and the like, in the case where ventilation holes with a high precision are formed therein, it is possible to uniformly supply gas. For example, according to the methods described in Patent Document 2 and Patent Document 3, it is possible to form ventilation holes with the same opening diameter in the front and rear surfaces of the electrode plate. However, it is necessary to conduct two steps, that is, the step of forming the pilot holes and the step of machining the inner surfaces of the pilot holes. Accordingly, there is a need for a machining method with higher productivity.